Xenotropic viruses were isolated in three of four experiments from tissues of the NIH Swiss mouse by direct inoculation of filtered tissue hemogenates into target cell cultures, demonstrating that infectious virus is produced in vivo under physiological conditions. Virus production occurred in only 20 to 40% of the individual mice examined. One such virus, designated M55, was selected as a representative NIH Swiss mouse virus for further use. Comparative analysis by peptide mapping demonstrated that the p30 major core proteins of different type C murine oncoviruses exhibit distinct structural features by which the viruses may be grouped into classes coinciding with those previously delineated by criteria not involving p30. The avian agent reticuloendotheliosis virus (REV), currently classified as a low risk agent, was shown to replicate in cultured mammalian cells. As part of a study of antiviral agents, interferon has been shown to act at a post-transcriptional step in inhibiting oncovirus replication. On site assistance in implementation of biohazard containment procedures was provided to investigators at more than 25 VCP collaborating laboratories.